472 T. W. E. DAVID, W. F. SMEETH, AND J. A. SCHOFIELD. 
marked, and the felspar granules are seen to consist largely of 
microcline. The hatched twin lamelle are very irregular and 
wavy and suggest that the rock has been under pressure. There 
isa small amount of triclinic felspars which appears to lie between 
albite and oligoclase. One section of the latter shows a remark- 
able twin structure (Plate 13, fig. 1), the outline is rudely rec- 
tangular and the grain is divided into four equal areas by lines 
joining the middle points of the sides. Each area is finely lamellar, 
and extinguishes simultaneously with the one diagonally opposite 
to it. A possible explanation may be that it is a section approxi- 
mately parallel to the macropinacoid (100) divided into two halves 
according to the Carlsbad law, and again divided into two halves, 
one of which is reversed on the other by turning round a normal 
to 001, according to the Mannebach law. In addition to this 
each of the four areas exhibit albite lamellation. Some of the 
felspars show numerous fine parallel cavities probably of the nature 
of solution planes. 
The quartz grains have some liquid enclosures but not many, 
and also some very tine needles of a mineral which has not been 
‘determined. Stray flakes of muscovite have been noticed. 
Among the accessory minerals the garnets do not require 
further notice. The tourmaline occurs in well shaped prisms the 
pleochroism being pale pinkish-brown to very dark slaty-purple. 
Included in the other minerals are numerous highly refracting 
grains and prisms, all practically colourless. Some of these must 
be referred to topaz, although the optical sign appears to be 
negative instead of positive, as is more usual. One piece in pat 
ticular, (Plate 13, figs. 2a, 2b) has the outline of a cross section 
of topaz and gives a good figure in convergent light. In this, 
from the shape of the section, it can be seen that the optic axial 
plane is the macropinacoid, and that the sign is negative while 
the axial angle is somewhat smaller than usual. If this mineral 
is a topaz, it would appear that in addition to the change of sign, 
the plane of the optic axes has taken a position at right angles od 
the normal one. Itisan interesting point in connexion with this, 
